Depositional systems in the Mt Isa Inlier from 1800 Ma to 1640 Ma: Implications for Zn–Pb–Ag mineralisation

Abstract

Two facies models are proposed to explain siliciclastic and carbonate depositional systems of 1800 Ma to 1640 Ma age in the Western Fold Belt of the Mt Isa Inlier. Both models record the response of depositional systems to storm-driven processes of sediment transport, dispersal and deposition on a shallow water shelf. The same suite of facies belts can also be identified in sedimentary successions of the Eastern Fold Belt. Slope driven processes of sediment transport and dispersal characterise turbidite and debrite deposits of the Soldiers Cap Group and Kuridala Formation and provide evidence for significantly greater water depths in this part of the basin from ca 1685 Ma. Through the recognition of unconformity surfaces, their correlative conformities, maximum flooding and ravinement surfaces, the facies belts are packaged into seven supersequences for the interval ca 1800–1640 Ma. The new correlations are shown in an event chart that correlates linked depositional systems across the entire Mt Isa Inlier. Thick successions of turbidite and debrite deposits are restricted to the eastern parts of the Mt Isa Inlier and do not occur in the Western Fold Belt. A major phase of extension and rifting commenced at ca 1740 Ma and by ca 1690 Ma led to significant crustal thinning and increased rates of accommodation over an area east of the Mount Dore Fault and Burke River Structural Belt. In the Mitakoodi Anticline and Kuridala-Selwyn Block, the rapid transition from shallow-water shelf depositional systems of the Prize Supersequence to significantly deeper-water slope environments of the Gun Supersequence coincided with the development of a platform margin, the deposition of turbidite and debrite deposits in deep water on the continental slope and the intrusion of mafic sills and dykes. Turbidite and debrite depositional systems of the Soldiers Cap Group and Kuridala Formations are restricted to a lowstand wedge of siliciclastic facies deposited basinward of a platform margin. Basin geometries and sediment architectures associated with this extensional event and recorded in the Gun Supersequence (ca 1685 Ma to 1650 Ma) provide an explanation for the geographic separation and fluid evolution pathways responsible for the Mt Isa Type and Broken Hill Type Zn–Pb–Ag deposits.